1. Field of the Invention
The present invention concerns a magnetic resonance apparatus of the type having a scanner with a basic field magnet, a stationary gradient coil unit, an antenna system for acquisition of magnetic resonance signals and a second gradient coil unit that can be introduced into an examination chamber of the scanner.
2. Description of the Prior Art
For specific magnetic resonance examinations, insert gradient coils (which are also designated as local gradient coils) are introduced into the existing gradient coil of an MRI scanner that is used for whole-body examinations. In comparison to whole-body gradient coils, these insert gradient coils are advantageous with regard to inductance, slew rate, the maximum possible gradient field strength and the stimulation of the patient within smaller external diameters. Insert gradient coils are used when only a specific body part, for example the head of a patient, is to be subjected to a magnetic resonance examination.
Such a second gradient coil unit, the insert gradient coil, is normally manually inserted into scanner. An insert gradient coil, however, can weigh up to 250 kg, such that the handling thereof and the mounting and removal thereof can be cumbersome and complicated.
A magnetic resonance apparatus with a movable gradient coil unit is disclosed in DE 101 41 405 A1. The gradient coil unit is designed as an additional gradient coil that can be introduced into the tube-shaped examination chamber of the first gradient coil unit. For this purpose, in the region of the examination chamber a support device with rails is provided in order to move the second gradient coil unit into the examination chamber as needed and to remove it again after the conclusion of the examination. It is disadvantageous that the guides and rails for the support device are permanently located in the examination chamber even when the second gradient coil unit is not used.
An insertion device for gradient coils is known from DE 102 29 489 A1. The local gradient coil is mounted on a boom that is fastened on a movable carrier unit arranged outside of the magnetic resonance apparatus and can be displaced relative to the carrier unit. This movable carrier unit has a not-insignificant space requirement, such that the accessibility to this side of the magnetic resonance apparatus is limited.